1. Field of the Invention
The invention relates to resistance spot welding, and refers more specifically to a structure for and method of effecting resistance spot welding with small, light weight welding structure which may include dual, mirror image transformers having closely spaced apart unitary secondary circuits which may include integral electrode portions, utilizing synchronized high frequency, electrical resistance spot welding signals which reinforce each other to provide push/pull welding, which transformers are positioned on opposite sides of work pieces to be welded immediately adjacent welds to be effected and are moved toward and away from work pieces during welding whereby large, long leads between the welding transformers and welding electrodes are eliminated with consequent increase in efficiency of the welding.
2. Description of the Prior Art
In the past, resistance spot welding has usually been conducted at the normal 60 hertz, power line frequency. At such frequencies, the transformers utilized in the past have been quite large and heavy. Such welding transformers often weigh one hundred pounds or more each.
The size and weight of such equipment greatly reduces its usefulness in that it prevents it from being close to the weld area. Wherein transformers have been located at a substantial distance from a weld area, extra long conductors have been used in the past between the welding transformer and the welding electrodes.
Further, spot welding systems of the past have generally included a single transformer used to supply current to two welding electrodes positioned to apply current to work through which electrodes a forging force has been applied to push the work pieces to be welded together for welding at a desired temperature.
With such structure, two conductors of some sort are needed to carry welding current between the transformer output terminals and the welding electrodes. Because resistance welding requires high currents (from 5,000 to 50,000 or more amps), the conductors carrying the welding current must be large in cross section and made out of a good conductor of electricity, such as copper. This requirement makes resistance welding apparatus heavy, difficult to move around, and expensive. Often, water cooling for these conductors is required, further adding to cost and complexity.
These conductors may also help supply a rigid frame work to allow force to be applied to the welding electrodes while holding them in alignment with each other, an important consideration in resistance welding.
Because the welding electrodes must be movable in relation to each other to apply the required forging force, at least one of the conductors needs to be flexible in order to allow for such movement through some type of flexible or sliding joint. If a weld is desired in an area some distance from the edge of a work piece, the conductors need to be long enough to react across to the weld area. Wherein the work to be welded is moving, as on an automobile assembly line, the conductor between the transformer and the welding tips is a large, flexible cable.
The conductors connecting the welding transformer to the electrodes in prior resistance spot welding systems have been a significant source of inefficiency, particularly when they need to be long because of a large work piece and wherein flexible cables are used. It is not uncommon to have more than three quarters of the total energy coming from the welding transformer wasted as heat in the conductors due to resistive and conductive effects. In some applications, the voltage coming out of the welding transformer can be reduced as much as ninety five percent before it reaches the weld.
Also, wherein the area to be welded has been surrounded by a number of obstacles, special shaped conductors and/or electrodes have sometimes been required in the past. These special shaped members have been required to be conductive and strong so that in the past they have usually been metal, which has increased their cost and weight.
Further, the conductors between the welding transformer and electrodes in prior resistance spot welding systems have been a significant source of maintenance problems. The most troublesome area of the conductors has been the point where they flex to allow movement of the welding electrodes. Moving joints which carry large currents at such points cause machine breakdowns, as moving parts wear and fatigue.
A prior art welding structure, such as referred to above, is shown in FIG. 1. The welding structure 11 includes a generally C-shaped, heavy metal frame 13, and transformer 15 secured in a fixed position on the metal frame. The transformer 15 may include primary winding 17, a secondary winding 19 and a core generally indicated 21. Welding electrodes 23 and 25 are connected to the secondary winding 19 of the transformer 15 through conductors which include the upper platin 27, flexible conductor 29 and conductor 31, and lower platin 33 and frame means 35. As shown, the frame means 35 is movable vertically in a slide 37 on frame 13, in accordance with the adjustment of the knee support 39. The platin 27 is supported for vertical movement during welding on the ram 41 by means of the air or hydraulic actuating cylinder 43.
Thus, in operation of the prior art welding machine 11, as shown in FIG. 1, a 60 hertz electrical welding signal is applied to the primary winding 17 of the transformer 15, work pieces not shown are positioned between the electrodes 23 and 25, the platin 27 is moved downwardly as shown in FIG. 1 with the ram 41 on actuation of cylinder 43 while the conductor 29 flexes. On completion of a circuit through the work pieces between the welding electrodes 23 and 25, a high current, low voltage welding signal is passed through conductors 31, 29 and 27, the electrode 23, work pieces not shown, welding electrode 25 and conductors 33, 35 and 37 back to the transformer secondary 19. Such structure and operation is subject to the deficiencies for such apparatus of the prior art as set forth above..